For example, as a social need being imposed on automobiles, an improvement is intensively required in fuel economy. This is effectively to be achieved by light-weighting. To the end, while aluminum and plastic materials have been being adopted, strength and precision tend to be incompatible to attain, thus becoming an acute technological problem. Also, while with an uprush of eco-consciousness in recent years, the bicycle industry proves active, enhanced light-weighting and also rises in strength and other mechanical properties and improvements in sense of quality are here, too, sought to differentiate products, giving rise to the foregoing problem. And, in the fields of electronic devices and others as well, enhanced light-weighting and rises in strength and other mechanical properties and improvements in sense of quality are being asked for.
As a technology to achieve light-weighting (i.e. thinning) and to improve strength and other mechanical properties, a semisolid casting technique is presently known.
The semisolid casting technique includes rheocasting and thixocasting methods.
Rheocasting is a method which comprises cooling an alloy from its liquid state while it is being agitated to grow primary crystal in the form of particles, followed by molding it when a certain rate of its solidification is reached. It is also called semisolid die-casting.
In the thixocasting method on the other hand which is also called semi-melt casting, an alloy molten is once solidified while it is being agitated to form a billet which then, when cast, is heated again to form a body in a solid and liquid coexisting state, the body being then molded.
The thixocasting method has a problem that a special billet of which the structure is adjusted is expensive. It has also a problem that it lacks energy saving since a billet is re-molten to form a semi-metal slurry for casting. Furthermore, the thixocasting method has a problem that a material that is once cast thereby cannot be re-molten for use and cannot be recycled. Hence, the rheocasting method is presently the mainstream.
There is a process in which after crystallization of a given amount of solid phase, a slurry in a solid and liquid coexisting state is teemed or poured into an injection sleeve for injection filling (NRC process: Ube's New Rheocasting Process; see, for example, Patent Reference 1).
The NRC process, however, requires time in forming the semi-solidified slurry, necessitates large and costly equipment and has a limit in micronizing a spherical crystal due to an insufficient number of occurrences of nucleation.
As a technique to break through the limitation, i.e., as a technique to form a slurry inexpensively, quickly and simply and to increase the number of occurrences of nucleation, there have been proposed a nano-casting process (Patent Reference 2) in which agitation is produced electromagnetically and a cup process (Patent Reference 3) by self-agitation.
Subsequently, problems of micronization of a spherical crystal have been combated to optimally control the temperature of a metal melt when teemed into a sleeve, leading to the development of a semisolid slurry forming process (Patent Reference 4) which without having the conventional slurry forming equipment allows a number of crystalline nuclei to be crystallized in the sleeve and by adequately controlling the crystal growth permits a microfine spherical crystal to grow which could not so far be grown in rheocasting.
In the meanwhile, of melt forging techniques to forge a metal melt in a die, ones using a rheocasting and a thixocasting method have been proposed, as described e.g. in Patent References 5 and 6, respectively.
In the technique described in Patent Reference 5, a massive mixture (billet) in a semisolid state is placed centrally of a lower die heated at a temperature lower than that of the massive mixture. Then, moving an upper die closer to the lower die allows the massive mixture in the semisolid state to be compressed and deformed.
The technique described in Patent Reference 5 has a problem, however, that the mass of a raw material is large compared with that of a product, making it costly. It should be noted here that the “mass of a raw material” refers to the mass of a raw material supplied into the lower die, and that the “mass of a product” ought to mean the mass of portions excluding an excess bur and any other part than the product. Also, both the masses of raw material and product are those at room temperature.
Further, in the case of a product having a thin portion (e.g. a portion of 1 mm or less thick), the thin portion needs to have a flash or an excess thickness added thereto which needs to be cut away subsequently in an additional process step, becoming a cause of making the process costly.
Patent Reference 6 (JP H04-182 054 A) describes a melt forging technique in which a melt of metallic material teemed into a press die is held therein for a fixed period of time in the state that it is under a pre-load, and an additional pressure is applied to at least a portion of the metallic material for a time interval from the start to the end of its solidification until its temperature is reduced to 300° C. to deform it.
However, the technique described in Patent Reference 6 needs to have a plurality of process steps of applying a preload and an additional pressure, rendering the process complex while having no choice but to complicate an apparatus therefor.
Also, Non-patent Reference 1 discloses a technique in which a semisolid slurry formed in a metal container shaped to follow a product shape is cast into a die for compression molding.
While this process allows obtaining a spherical structure, the process requires steps in which a semisolid slurry is once prepared and is then transferred into a die. Furthermore, a mass of raw material is made larger than that of a product, making the technique costly from the aspect of raw materials.